As semiconductor processes become minuter, in recent years the absolute value of power-supply voltage has become smaller and the requirements of accuracy with which voltage is set have become severer. In addition, with an increase in integration level, there is a tendency for power-supply current to increase to several tens of amperes.
POL (Point Of Load) by which a power-supply circuit, such as a non-isolated step-down DC(Direct Current)-DC converter, is placed near a device that is the load on a power supply is effective for systems having the above uses. By adopting the power supply architecture of the POL, a loss caused by a drop in voltage is prevented or responsibility is improved.
In order to accommodate fluctuations in current consumed by LSI (Large Scale Integration), it is desirable with the POL to make a switching frequency of a switching power supply as high as possible. However, if a switching frequency is too high, great loss occurs in a switching element such as a FET (Field Effect Transistor).
A technique referred to as soft switching is known as a technology for preventing such loss at switching time and stabilizing output voltage. A resonant converter is widely known as typical soft switching.
For example, the following two techniques were proposed formerly as a resonant converter. One is to divide resonance capacitors and perform switching for changing a resonance frequency. By doing so, the resonance capacitors have plural values according to switching frequencies. The other is to change a resonance frequency by changing an inductance value.
Japanese Laid-open Patent Publication No. 01-248957
Japanese Laid-open Patent Publication No. 09-201044
When disturbance, such as fluctuations in load, occurs, control is exercised for stabilizing output voltage. If a resonance frequency is variably set at this time, circuit structure is complex with the above conventional resonant converters.
In order to stabilize output voltage, control is exercised for switching a resonance capacitor under the output condition or the input condition under which the value of a switching frequency is smaller than or equal to a limit value (see, for example, Japanese Laid-open Patent Publication No. 01-248957).
However, a switching frequency (switching timing) of a switching power supply is sequentially made to match switching timing of the resonance capacitor in an adaptive manner during operation. As a result, circuit structure in a control system is complex.
In addition, a transformer-type inductor is used and a secondary side of the transformer is switched (see, for example, Japanese Laid-open Patent Publication No. 09-201044). As a result, great loss occurs on the secondary side and a control circuit for preventing this loss is newly placed.